The iambic trimeter derives its name from its essential shape, which is three metrical units (hence trimeter) which are each basically iambic in form. The iambic metron has the following shape (where the x is an anceps, the - is a longum, and the u is a brevis):
Hence, reversing the current running though a wire will have the affect of reversing the direction of the magnetic force at a point some distance away from the wire. The affect that this has on the overall magnetic field depends on the shape of the wire.
The direction of this field can be determined by using your right hand with your thumb pointing outwards and wrapping your hand around the wire. The direction your thumb is pointing is the direction of the current, and the direction your fingers are wrapped around the wire are the direction of the magnetic field.